"Were they worried it was going to slip down the hill?"
I am not sure, but I believe that there has, for many years, been a restriction on trains leaving Central until the platform at St Davids is empty. If I am right, then it is not a new fear.
I think I saw a comment somewhere that the worry was gauge clearance with respect to the platform ends at Exeter St David’s for trains to/from the curve. Can’t remember where I saw it though.
Paul